Minimally invasive surgical techniques are known for performing medical procedures within all parts of the cardiovascular system. Exemplary known procedures include the steps of passing a small diameter, highly-flexible catheter through one or more blood vessels and into the heart. When positioned as desired, additional features of the catheter are used, in conjunction with associated equipment, to perform all or a portion of a medical treatment, such as vessel occlusion, tissue biopsy, or tissue ablation, among others. Almost always, these procedures are performed while the heart is beating and blood is flowing. Not surprisingly, even though visualization and positioning aids are adequate for general placement of the device, maintaining the device in a selected position and orientation can be difficult as the tissue moves and blood flows, especially during a procedure that must be done quickly. As diagnostic and visualization equipment and techniques have continued to evolve, it has become possible to identify tissue areas to be treated with greater precision than the ability to quickly situate the device and effectuate treatment.
In addition to the challenges presented by moving tissue and flowing blood, the actual topography of the tissue being treated presents challenges. For example, unlike stylized drawings that depict the interior of the chambers of the heart as having smooth, evenly curved walls leading neatly to tubular blood vessels, the interior surfaces of the heart's chambers are irregular, uneven, and fibrous, as are the openings to blood vessels. Thus, for procedures that call for uniform tissue contact or tissue contact along an extended line, the structure and techniques for use of known devices can be deficient in some regards.
Even if a device is capable of being properly placed and held in position at the proper orientation; and even if the device is suitable for the tissue topography at the treatment site, the device can be nevertheless not fully suitable to achieve the desired outcome. By way of example, catheter-based devices are known for placement in the left atrium for ablating tissue within the atrium for the purpose of electrically isolating one or more pulmonary veins from the atrium in an attempt to increase the success rate of atrial fibrillation ablation.
In one type of prior art device disclosed in U.S Patent Publication 2002/012836 A1, and as shown in FIG. 1 (prior art), a sheath or guide catheter 10 is inserted into a blood vessel 12 that leads to the right atrium 14 of the heart 16 and passed through an opening created in the septum 18 that separates the right and left atria into the left atrium 20. As shown in FIG. 2 (prior art), a treatment element 22 is passed through the guide catheter 10, deployed from an opening in the distal end thereof, and caused to form a substantially circular loop that is traverse or perpendicular to the longitudinal axis of the guide catheter 10. A distal tip element 24 that extends distally beyond the circular loop is inserted into a pulmonary vein 26 as a guide and placement aid for the loop. As shown in FIG. 3 (prior art), the treatment element 22 in the form of a loop is placed so that it encircles the opening or entry of the pulmonary vein 26, known as the ostium, and tissue is ablated by microwave heating of the contacted tissue. The intended result is a substantially uniform circle of ablated tissue 28 as shown in FIG. 4 (prior art). Also as shown in FIG. 4 (prior art), such a device can be used in an attempt to create linear lesions 30 and 32 as well.
In practice, uniform, unbroken lesion lines are hard to create with such loop shaped ablation elements. Also, with respect to both the circular and the linear lesions formed by microwave ablation, it should be noted that the lesion formed is relatively narrow and has a width that corresponds to about the width of the catheter. Devices that use a laser to ablate tissue provide a similar result; namely, a very narrow lesion. Further, because a laser ablates a very narrow line of tissue, precise alignment of the device is very important. However, for the reasons set forth above, such precision is very difficult to achieve.
In another type of device disclosed in U.S. Pat. No. 6,164,283 electrode elements, capable of ablating tissue when energized, are circumferentially disposed on the exterior of a balloon element that is placed at least partially within a pulmonary vein, so that the electrodes are positioned to form a circumferential conduction block along a circumferential region of tissue in a pulmonary vein wall. Other device configurations are disclosed that have an electrode band positioned on an expandable member for ablating a circumferential path around the ostium and tissue along the posterior atrial wall which surrounds the pulmonary vein.
Recently, companies such as CryoCath Technologies Inc., Montreal, Canada, have developed catheter based devices that cryogenically ablate tissue. These devices are structurally very different from RF catheter based devices, and they are not similar or comparable variations on the same theme. Not only are the structures that encompass the respective ablation technologies different, but so are the devices for controlling the ablation process, evaluating the progress and extent of ablation, and ensuring patient safety.
For example, to create a large “ring” with and RF catheter it is necessary to make a series of adjoining spot lesions of relatively small size using small electrode if one wishes to minimize RF output. This is significant because use of a large electrode and/or high power output can seriously injure tissue at other than the intended treatment site. This is especially important with respect to creating lesions in the pulmonary veins because the veins are juxtaposed with bronchial tubes and other sensitive pulmonary tissue within which it is highly undesirable to create ancillary lesions. By contrast, cryogenic ablation of tissues does not need to be accomplished “bit by bit” for fear to transmission of energy into tissue and the transfer of heat occurs at the medical device.
Another disadvantage common to RF and other non-cryogenic devices that was identified above is the difficulty of maintaining such a device in a selected position while the heart is beating. By contrast, a cryogenic device does not have this problem because the subfreezing temperatures created by the device causes the device to firmly stick or adhere to tissue at a treatment site. Still further, RF energy causes cellular death and side effects that are specific to the use of RF energy and that contrast considerably with the effects of cooling and the cellular death caused by freezing tissue.
Thus, although RF ablation may be appropriate and safe for tissue ablation in other areas of the body, clinical studies have revealed that creating of RF lesions in the pulmonary veins does not appear advantageous for the reasons set forth above. Further, RF ablation of the pulmonary veins has been associated with left atrial-esophageal fistula, pulmonary vein stenosis, tamponade, and significant radiation exposure. Therefore, if a cryogenic device were available as an alternative to RF ablation devices and techniques for the treatment of atrial fibrillation, it would be preferable to use the cryogenic device to avoid the problems created by the use of RF energy.
Notwithstanding the apparent advantages of cryoablation over other types of ablation, particularly RF, with respect to treatment of atrial fibrillation, very few cryoablation devices have been conceived for this purpose. Or, if cryoablation has been considered, it is mentioned as if it were synonymous with RF ablation, with no actual thought or disclosure provided that is enabling for such a structurally dissimilar device or that takes into account the very different effects of the use of the respective devices on tissue. For example, although U.S. Pat. No. 6,164,283 makes a brief, generalized reference to use of a cryoablation element to cool tissue, no specific details are set forth with respect to such a device.
Only one known device addresses issues related to cryoablation technology with respect to an attempted treatment of atrial fibrillation. Specifically, a cryoballoon catheter provided by Boston Scientific Corporation, Natick, Mass., has been used to create cryolesions by delivering liquid N2O into a semi-compliant balloon positioned at the pulmonary vein-left atrial interface. Thus, the device has basically been used as a substitute for an RF device to perform substantially the same procedure that is set forth in U.S. Pat. No. 6,164,283; namely, the creation of a substantially annular ring of ablated tissue at the ostium of the pulmonary vein. Although this device may obviate the adverse effects of the earlier RF devices, it would be desirable to provide a cryoablation device that more fully exploits the inherent advantages of a cryogenic ablation device.